Laser diodes are used in many applications including materials processing, medical devices, telecommunications, printing, etc. In many of these applications, and in particular those in which laser diodes are used to optically excite or “pump” the gain media of other lasers, the laser diodes are aligned along a common substrate at regular intervals to form a row or strip of diode emitters. These laser diode bars provide a relatively high brightness.
For example, referring to FIG. 1, there is shown a perspective view of a prior art laser diode bar 100. In this example, the diode bar includes ten diode emitters 102. Each diode bar typically has a width 110 that is approximately 150 microns along an axis 116 that is perpendicular to an axis 112 of the diode bar stripe. The diode bar 100 emits a combined output from the combined emitting areas of the individual diode emitters 102. Each diode emitter 102 typically has an area with a width 108 and length 106 that are, for example, one micron and 100 microns, respectively.
Output beams 104 produced by the individual diode emitters 102 along an output axis 114 have a relatively broad angular divergence in one direction or axis 118 and a smaller degree of divergence in the orthogonal direction or axis 120. These axes are often referred to as the “fast” and “slow” axes, 118 and 120, respectively.
To provide increased brightness traditional multi-emitter pumps have been typically fabricated by stacking laser diode bars to form stacked laser diode bar assemblies (e.g., up to about 50 laser diode bars in one stack). More specifically, the beams emitted from the different diodes are collected in the same space to provide a spatial multiplexed output having a relatively high brightness. One example of a stacked laser diode bar assembly is shown in FIG. 2, wherein the slow axes are stacked above each other. More specifically, FIG. 2 shows a perspective view of a stack 300 of diode bars 301 with emitters 302 mounted in between cooling slabs 330 in a modular arrangement. Cooling passages or channels 332 and 334 are provided for liquid heat transfer. Spacers 340 are optionally provided to facilitate alignment of the diode bars 301 and cooling slabs 330.
One limitation of the attainable brightness of stacked laser bar assemblies, such as 300, is the spacing or “pitch” between laser bars in the particular stack. Referring to Eq. 1, the brightness B of a given light source, for example a laser diode or diode bar 301, is described as:B=P/(A*·Ω)  (1)
where, P is the power output of the particular light source, Ω is the solid angle of the beam divergence, and A is the area of the light source. The brightness of a given light source consequently includes a power component, an area component and a divergence component. Typical units of measure are Watts for P, steradians (ster) for Ωcm2 for A, and Watts/cm2/ster for B.
Eq. 1 shows that the brightness of a laser diode stack is reduced by the percentage of non-light-emitting area of the structure outside the diode emitters. The ratio of the total emitter area compared to the total area of a stack is sometimes referred to as the fill factor of the diode stack. In order to increase the fill factor and still provide convenient heat removal (i.e., from cooling slabs) laser diode bars have been provided on stepped support structures. For example, referring to FIG. 3 there is shown a side view of an embodiment 500 of a stacked array. The stacked array 500 includes two or more laser diode bars 502, with each positioned on a support structure 514 in a stacked relation to adjacent diode bars. The support structure 514 includes a substrate or support surface 504 and has steps 503 that support the diode bars 502. The support surface 504 acts as a heat spreader or conductor. The steps 503 typically also support one or more optics (not shown) for collimating the beams emitted from the emitters. For example, in one embodiment the steps 503 support a first plurality of collimating lenses (e.g., where each lens is disposed in front of an emitter) for collimating the beams along the fast axis, and second plurality of collimating lenses (e.g., where each lens is also disposed in front of the emitter, but farther down the beam's path) to collimate the beams along the slow axis. In addition, in traditional spatial-multiplexed multi-emitter designs, the collimated beam transmitted from the emitters typically is combined into an output fiber via a coupling lens. Collimated beams from individual emitters are parallel to the optical axis of the coupling lens. Furthermore, the output fiber is also aligned at the optical axis of the coupling lens.
In general, the step height (t) is influenced by the individual diode bar heights and the thickness of associated electrical or ohmic contacts. The step height (t) is related to the combined beam size in the fast axis direction (h) by:h=Nt  (2)
where N is number of emitters in a row. However, while a smaller step height can combine more emitters, it may also cause beam clipping, which results in a lower coupling efficiency (CE). In fact, it is typically necessary to balance the step height with the number of emitters to attain high coupling efficiency. If the number of emitters change, the step height should be re-optimized to achieve the best coupling efficiency. Unfortunately, it is impractical to create different packages for different target power levels (i.e., different number of emitters per pump).
It would be advantageous to provide a spatial-multiplexed multi-mode emitter pump that provides high coupling efficiency for a varying number of emitters.